#include <iostream>

using namespace std;

string str;

int main()
{
	cin >> str;

	int cnt = 0;
	int n = str.size();
	for (int i = 0; i < n / 2; i++)
	{
		if (str[i] != str[n - i - 1])
			cnt++;
	}

	cout << cnt << endl;

	return 0;
}